1. Field of the Invention
The present invention relates to vacuum gauges such as Bayard-Alpert (BA) ionization gauges and to systems and methods for operating and calibrating such gauges.
2. Discussion of the Prior Art
The BA gauge is the simplest known non-magnetic means of measuring very low pressures and has been widely used worldwide essentially unchanged since being disclosed in U.S. Pat. No. 2,605,431 in 1952.
A BA gauge system consists of a BA gauge and controller circuitry. A prior art BA gauge consists of a heated cathode for emitting electrons, a cylindrical grid or anode for accelerating the emitted electrons to ionizing energy, a very small cross section collector electrode on axis for collecting the ions formed within the anode volume by collision of energetic electrons with gas molecules and a vacuum envelope surrounding the gauge electrodes and attaching to a vacuum system wherein an unknown gas pressure is to be measured. Controller circuitry consists of a means to apply suitable potentials to the anode, to the cathode and to the collector electrode, means for heating the cathode to provide a controlled electron emission current and means for measuring the collector current and means for calculating and displaying the indicated pressure.
As a first order approximation, it can be stated that the ion current i.sub.+, to the collector electrode is proportional to the electron emission current i.sub.--, and to the gas density in the gauge or at constant temperature to the gas pressure, P.sub.G, in the gauge. Thus, EQU i.sub.+ =Si.sub.-- P.sub.G Eq. 1
where S is a constant of proportionality commonly called the gauge sensitivity.
S can be calculated by measuring i.sub.+ and i.sub.-- when the gas pressure P.sub.GK in the gauge can be determined by other calibration means. Thus, EQU S=i.sub.+ /(i.sub.-- P.sub.GK) Eq. 2
where P.sub.GK is known pressure in the gauge. The value of S thus determined can be utilized to calculate the value of an unknown gauge pressure P.sub.x which produces an ion current, i.sub.+x at the same value of i.sub.-- used to determine S in Eq. 2 provided conditions in the gauge do not change. Thus, EQU P.sub.x =i.sub.+x /(i.sub.-- S) Eq. 3
Very complex and costly non BA gauges have been described which are claimed to provide excellent accuracy of measurement over a limited pressure range. However, these laboratory devices are entirely unsuitable for everyday use in research and industry and have yielded few clues if any as to how to improve the accuracy of the simple prior art BA gauge.
Numerous studies over the years have demonstrated conclusively that S is not constant in prior art BA gauges. For example, Poulter and Sutton at the National Physical Laboratory in the United Kingdom reported that a typical prior art BA gauge calibration "drifted at a rate of -1.4% per 100 operating hours when it was kept under vacuum but similar gauges exposed to atmospheric conditions showed sharp changes of up to 25% in their sensitivity", K. F. Poulter & C. M. Sutton, Vacuum, 31, 147-150 (1981). In another representative study, Tilford at the U.S. National Bureau of Standards reported that typical prior art nude BA gauges (BA gauges without vacuum envelopes) had sensitivities which varied from 70% to 110% of that specified by the manufacturer, C.R. Tilford, J. Vac. Sci. Technol., A1(2), 152-162 (1983). In another study, repeated calibrations of seven prior art "broad range" BA gauges showed sensitivities varying from 52% to 67% of their specified values, C. R. Tilford, K. E. McCullogh, H. S. Woong, J. Vac. Sci. Technol., 20(4), 1140-1143 (1982).
Ionization gauge systems must be calibrated against a primary or secondary pressure standard to be useful. Users of prior art BA gauge systems have been provided with three alternatives to secure required accuracy of vacuum measurement.
1. Calibrate the ionization gauge system in situ utilizing, for example, an expensive spinning rotor gauge. Such a solution is being actively promoted now by several spinning rotor gauge suppliers. The prior art gauge may change calibration at anytime, so frequent in situ calibration is required. This alternative provides the best accuracy bu is extremely time consuming, inconvenient and costly. PA0 2. Send the ionization gauge system to a separate calibration facility and have the system calibrated--an expensive, time consuming alternative at best because the prior art gauge may change calibration anytime with use. This alternative enables the user to comply with certain governmental requirements but is unlikely to result in improved accuracy over the long term. PA0 3. Use the generic calibration data supplied by the manufacturer assuming it is accurate for the prior art BA gauge in use. This generic calibration data is typically obtained by measurements on one or more prototypes and consists of a fixed value of the sensitivity S for a given gas type and a list of the nominal applied electrode potentials. PA0 1. Glass envelope BA gauges PA0 2. Glass envelope BA gauges with conductive coatings PA0 3. Nottingham gauge PA0 4. Nude gauge with exposed feedthru insulators PA0 5. Nude gauge used with undefined enclosure geometry PA0 6. BA gauges with large tolerance electrode geometry PA0 7. BA gauges with electrode geometries that change with time. PA0 8. BA gauges with electrodes fixed relative to one another but with large tolerance between electrodes and enclosure.
The huge majority of users opt for the last alternative because of the large expense and inconvenience of either of the other alternatives even though using generic calibration data provides vacuum measurements grossly in error.
What is needed is better means for providing generic calibration data that will yield more accurate vacuum measurements.
Many researchers have pointed out for years that S is not a constant but depends on gas species, electron energies, emission current, electric field distributions, gas density, kinetic energy of the gas molecules in the gauge, plus several other parameters. But from all this work there have been no solid clues on how to improve the accuracy of the prior art BA gauge and the inaccuracy remains the same as when the device was invented. From the vantage point achieved through computer simulation of electron and ion trajectories in a BA gauge, it is now possible to discern deficiencies in the prior art which help cause the observed large inaccuracies in prior art BA gauge systems.
It is well known that the performance of BA ionization gauges can be seriously affected by a phenomenon known as surface ion desorption. An effective way of minimizing this effect is by utilizing an anode which has minimum surface area. Thus, to minimize surface ionization inaccuracies all low pressure BA gauge designs utilize transparent grids with minimal surface area. When such open grids are used, energetic electrons are not confined to the anode volume but travel a significant fraction of their total path length outside the grid. These energetic electrons traveling outside the anode volume can impinge on exposed insulating surfaces and uncontrollably change the surface potential of the exposed insulator. Their trajectories may also be changed by uncontrolled potentials outside the anode volume. Utilizing computer simulation, one can determine if energetic charged particles will impact an exposed surface or if trajectories are influenced by potentials outside the anode volume for any given configuration of surfaces, potentials, energy and initial trajectory of the charged particles.
The total electric charge distribution in a BA gauge is the sum of the charge distribution on the surfaces exposed to impact by charged particles and the charge distribution due to free charges within the gauge volume. In prior art BA gauges, means have not been provided for adequately fixing the electric charge distribution in that region of the gauge accessible to energetic charged particles. Thus, the electric charge distribution can vary from measurement-to-measurement in the same gauge or from gauge-to-gauge at any given pressure. Thus, because the electric charge distribution which exists during calibration cannot be duplicated during use, inaccurate pressure indications result.
Applicants have found by computer simulation that seemingly trivial changes in the electrode geometry or in surface potentials in the gauge volume accessible to energetic charged particles can cause large changes in electron trajectories. It is well known that changes in electron trajectories cause changes in the radial position, r.sub.o, of ion formation resulting in changes in the angular momentum, m r.sub.o v.sub.T, of the ion, where m is the mass of the ion and v.sub.T is the tangential component of its velocity. Changes in angular momentum cause the probability of ion collection to change as is well known, thus causing the gauge sensitivity to change. Changes in gauge sensitivity cause measurement inaccuracies.
Prior art BA gauges can be categorized as to the manner in which and the degree to which the electric charge distribution on surfaces exposed to charged particle impact in the gauge are uncontrolled.
The huge majority of BA gauges in use have glass vacuum envelopes where the entire inner surface of the gauge is exposed to energetic charged particles. Energetic electrons and ions can impact on these insulator surfaces and charge them to uncontrolled potentials. Thus, in glass gauges the electric charge distribution on the insulator surfaces is totally uncontrolled. Abrupt changes in indicated pressure of 50% or more are regularly observed at 10.sup.-5 Torr or higher particularly at lower emission currents.
Conductive coatings such as stannous oxide and platinum have been deposited on a portion of the inner glass wall of BA gauges and held at a fixed potential, as disclosed, for example, in U.S. Pat. No. 3,839,655. However, in all such prior art devices the glass insulators and substantial areas of the inner glass surface have remained uncoated and exposed to energetic charged particles. Thus, the exposed glass insulator surfaces can charge up uncontrollably and change the electric charge distribution in such prior art devices. Applicants have found that changes in port diameter or even in the length of the port tubulation affect the charge distribution in prior art devices.
Nottingham at M.I.T. positioned a screen grid between the glass wall and the BA gauge electrodes but did not enclose the ends of the screen grid, W. B. Nottingham, AVS Vac. Symp. Trans. 8 494 (1961). The anode ends were enclosed with grids but the screen grid ends were left open. Thus, the exposed glass surfaces on both ends of the gauge can charge up uncontrollably and change the electric charge distribution in the gauge.
So-called nude gauges have the electrode assembly mounted on a metal flange which is penetrated by multiple feedthru insulators. In this regard, see, for example, Varian Associates Series UHV 24 Specification Sheet. The collector electrode feedthru is invariably covered with a metal shield to prevent soft x-rays from impinging on the collector electrode support thus decreasing the indicated base pressure of the gauge. The remaining feedthru insulators are exposed to energetic charged particles and can charge up uncontrollably and change the electric charge distribution.
Nude gauge electrode assemblies are intended to be inserted into a metal vacuum system. The interior surfaces of the vacuum system surrounding the electrode structure help to define the electric field configuration in regions traversed by electrons. A nude gauge is typically calibrated in one vacuum system geometry and used in a different geometry or even exposed to different potentials. The electric charge distribution in such cases is different during use from that present during calibration. Thus, the indicated pressure cannot be accurate.
The American Vacuum Society Standard 6.4, "Procedure For Calibrating Hot Filament Ionization Gauges Against A Reference Manometer In The Range 10.sup.-2 -10.sup.-5 Torr" contains no directives, cautions, nor any hints that the electric charge distribution must be controlled to achieve accurate pressure indication.
In AVS Standard 6.4 there is a note: "The position of the tube relative to the chamber wall and the temperature of the chamber wall exposed to the tube are factors in the gauge calibration and, hence, should duplicate actual usage as nearly as possible". This note is made in the context of ensuring that all gauges are symmetrically placed so that gas temperatures and conductances are the same for the test gauges and reference manometers.
U.S. Pat. No. 3,742,343 discloses a Groszkowski gauge wherein a cylindrical electrical conducting screen is described for use in a nude ion gauge to ensure repeatability of pressure measurement in various configurations of the vacuum system. However, the screen described does not completely enclose the end of the screened region. Thus, variations in the electric charge distribution on unshielded exposed surfaces can affect the sensitivity.
In 1986 Hseuh and Lanni at Brookhaven National Laboratory tested 400 custom nude BA gauges in different specified diameter metal envelopes and observed considerable variation in sensitivity S, see H. C. Hseuh & C. Lanni, J. Vac. Sci. Technol. A5 (3) Sep/Oct 1987, p.3244. Neither screens on the port end nor insulator shields were present during these tests. Thus, the electrical charge distribution on exposed surfaces could vary during testing, thus affecting the sensitivity.
In a recent research paper, Peacock and Peacock reported on tests on nude gauges with and without closed ends on the anodes, see R. N. Peacock and N. T. Peacock, j. Vac. Sci. Technol., A Vol. 8, No. 4, Jul/Aug 1990. The authors state, "To obtain reproducible conditions all nude ionization gauges were operated in nipples 97 mm long by 33 mm inside diameter. These were provided with a screen at the chamber end of the nipple to prevent ion coupling between gauges. The tubulated glass gauge had a similar screen on its flange." However, no insulator shield is provided on the tested gauges.
In addition to the foregoing considerations, to the best of applicants' knowledge, no prior art nude gauge manufacturer nor distributor has provided the pertinent dimensions of the controlled environment used to provide a generic calibration of the gauge. Thus, the user has no means to duplicate the generic calibration geometry during use and the pressure indications are therefore likely inaccurate.
A cursory examination of any ten commercially available prior art BA gauges with the same part number will reveal significant dimensional differences in electrode positions. Variations of as much as 0.25" are evident in some products. Because the electric charge distribution is not at all the same in such gauges as in the prototype gauge which was actually calibrated by the manufacturer, large inaccuracies in indicated pressure can be observed when the generic calibration data is used.
Designers of prior art BA gauges have failed to take the necessary precautions to maintain all electrode positions constant with use. Most glass BA gauges utilize untensioned and unsupported hair pin shaped cathodes which typically sag and move by tenths of inches in long term use. Bifilar wire anodes sag and warp in long term use. Ion collectors that are not adequately stress relieved can change shape over time especially when subjected to high temperatures during electron bombardment degassing. In some nude gauges the collector electrode position changes with externally applied force on the feedthru pin. Bimetallic joints cause electrode motion as the power input into the gauge varies. In some prior art BA gauges the collector electrode is so long and flimsy that in use it vibrates with a relatively large amplitude that changes with external conditions. Because the electric charge distribution changes uncontrollably with time in such gauges, the gauge becomes inaccurate.
In many prior art gauges it is common to observe relatively large variations in the location of the entire electrode structure relative to the vacuum envelope. Such variation, of course, affects the electric charge distribution in the volume accessible to energetic electrons. Changes in the electric charge distribution cause inaccuracies in the indicated pressure gauge to gauge.
Thus, the situation with prior art BA gauge systems is as follows: The electric charge distribution on surfaces in prior art gauges is so ill-controlled that there is little point in providing generic calibration data for a given gas type other than (1) a constant value for the sensitivity for use at all pressures, (2) nominal values of potentials applied to the cathode and anode relative to ground, and (3) a nominal value of the emission current used during calibration.
Thus, even though it has long been known that the BA gauge calibration depends on a number of variables, it has not been possible to correct the indicated pressure in a prior art BA gauge system for changes in any or all of the variables.
What is needed is a better approach to providing a more accurate BA gauge system.